Optical fibers are normally made in a process which involves drawing a thin glass strand or fiber from a partially molten glass preform, and thereafter coating the strand of fiber with a polymer to preserve its structural strength. The composition of the preform will largely depend upon the type of optical fiber desired (single mode, multimode, dispersion shifted, etc.)
Glass is an amorphous material, which can be brought to a low viscosity condition by heating. With a quartz glass preform normally used to manufacture optical fibers, a significant softening of the glass occurs at a glass softening temperature of about 1900.degree.to 2200.degree. K. Glass softening at the glass softening temperature is employed to enable the drawing of the preform into a thin glass fiber. To that end, the preform is heated in a drawing oven (furnace), which can be a graphite resistance furnace wherein a heating element is brought to the glass softening temperature by pulsating direct or alternating current. Alternatively, an induction furnace may be used wherein a tube, made of zirconium oxide or graphite, is brought to the glass softening temperature by an electro magnetic field.
The production of optical fibers typically occurs on what is known as a drawing tower, which is a vertically oriented production device for manufacturing optical fibers. The furnace is located at the top of the drawing tower for heating the glass preform, and the strand or fiber drawn from the preform passes through various measuring, cooling, coating and curing stages prior to passing through a tensioning and drawing device located at the bottom of the drawing tower. The tensioning and drawing device is typically a capstan arrangement wherein the optical fiber is fed off to a take up spool.
The heat of the furnace and the rate of draw of the fiber must be in proper balance so that the fiber can be drawn continuously with a uniformity of desired properties. This balance is normally accomplished by monitoring the tension of the coated fiber while drawing the fiber at a uniform rate, typically between 1 and 10 meters per second. If the coated fiber tension rises above a prescribed value, the heat of the furnace is typically raised which results in a reduced tension. Similarly, the furnace temperature is lowered in response to the tension falling below a prescribed range of values. It is also known to control the fiber tension by controlling the draw speed. For example, if the tension of the coated fiber is too low, the draw speed may be increased to increase the fiber tension. If the tension in the fiber is above a prescribed range of values, the fiber draw speed may be reduced to thereby reduce fiber tension.
One known drawing mechanism is a capstan mechanism which includes a pair of capstan wheels held in fixed position with respect to one another wherein the fiber is fed between the capstan wheels. One or both of the capstan wheels may be coated for example with a rubber coating, to firmly grip the fiber. Another known capstan arrangement includes a motor driven wheel of, for example, 30 centimeter diameter, against which the coated optical fiber is pressed by a capstan belt. The fiber may contact the surface of the capstan over a portion of the circumference equal to, for example, 90.degree. (1.6 radians). After the capstan, the fiber may pass over one or more pulleys before being wound onto a take up spool. The capstan belt serves to insure sufficient contact between the fiber and the draw capstan such that the fiber does not slip against the capstan and such that the capstan completely controls the speed of the fiber throughout the process. Therefore, there is a significant pressure placed by the capstan belt on the fiber against the capstan wheel. In the industry today, capstan belts are typically fiber re-enforced hard rubber, or may be tough woven fabric with a significant weave pattern, pressing against the fiber as it passes through the capstan. These materials ensure good wear and long service life of the belt.
It has been found that when utilizing higher draw speeds, the fiber coatings are still at a relatively high temperature (up to 100.degree. C. or higher) when the fiber enters the capstan. When a pair of capstan wheels are used to draw a coated fiber by passing the coated fiber between the capstan wheels, the coated fiber is "pinched" resulting in deformation of the fiber coating in pressing it against the capstan wheel. Similarly, in the single capstan and belt arrangement, the hard rubber or woven capstan belts deform the fiber coating in pressing it against the capstan wheel. At the same time, the coating is cooled through contact with the steel capstan to below its glass transition temperature, thereby freezing the deformation into the coating. The deformation in the coating translates to an inhomogeneous stress distribution on the glass fiber which results in microbending-induced added loss. With time, the deformation relaxes out of the secondary coating to some extent which reduces the added loss. However, the degree of added loss reduction is variable with permanent added loss observed up to 0.1 dB/KM in some cases. Therefore, a need exists in the art for a method and apparatus for drawing an optical fiber which minimizes deformation of the fiber coating during fiber drawing.